The invention relates to a sputtering cathode on the magnetron principle, having a target of the material to be sputtered which consists of at least one piece, having a magnet system disposed behind the target with a plurality of magnet units of alternating polarity lying one within the other by which at least two endless magnetic tunnels of arching lines of force are formed which lie one within the other, while the poles of the magnet units facing away from the target are joined together by a magnet yoke of soft-magnetic material.
DE-OS No. 34 42 206 discloses a sputtering cathode of the kind described above, which is intended for the sputtering of targets of ferromagnetic materials. In one of the two embodiments, two endless magnetic tunnels of arcuately curved lines of force are formed, one within the other, by two magnet units lying concentrically one within the other, in conjunction with two concentric air gaps disposed one inside the other. But since in this case the magnetic fields above the two air gaps are magnetically in series, the field strengths of the two tunnels cannot be varied independently of one another. Thus, different sputtering rates develop within the two magnetic tunnels, as well as very different rates of deposition on the substrate confronting the target, so that the thickness of the deposited coating is very irregular.
It is known through DE-OS No. 22 43 708, in the case of bar cathodes, to dispose magnet units one behind the other in the axial direction in order thereby to make the sputtering of the target material uniform. Even in the case of the planar targets therein described it is stated that concentric magnet units lying one within the other and a plurality of magnetic tunnels lying concentrically one within the other can be provided. Here again no provision is made, nor is any possible, for a separate adjustment of each magnetic tunnel to be performed independently of the adjacent tunnel.
Through a sputtering cathode of the kind described above, which is sold by Varian under the name "Con Mag", it is furthermore known to provide a flat circular yoke plate within a flat annular yoke plate with a gap between them, and to associate with each yoke plate two magnet units of opposite polarity, acting independently of one another, while a target having a conical sputtering surface is disposed above the circular magnet system and a planar target plate is disposed above the circular disk-shaped magnet system. No means is provided for varying independently of one another the magnetic field strengths of the two tunnels that are formed. In this sputtering cathode the sputtering effect must be accepted as it is created by the relative position of the magnetic tunnel in relation to the sputtering surfaces.
Now, the distance between the magnet system or systems and the target has a considerable effect on the distribution of the lines of force over the sputtering surface and hence on the so-called "erosion profile", especially when the target consists of a nonferromagnetic material. To compensate the increasing consumption of the target material by the formation of an erosion pit, it is proposed in DE-OS No. 30 47 113 to vary the distance between the magnet system and the back of the target as the consumption of the target material progresses. The magnet system in question, however, consists of only two magnet units which produce a single magnetic tunnel, so that this disclosure, again, contains no suggestion for controlling the coating thickness distribution when two or more magnetic tunnels are used, lying concentrically one within the other.
Furthermore, when a single endless magnetic tunnel is used, the distribution of the thickness of the coating on the substrates depends also to a great extent on the distance between substrate and target. Measurements have shown that, if the distance between the target and the (stationary) substrate is too great, an excessively great coating thickness is to be observed at the center of the substrate, decreasing greatly toward the margins. If the distance between the target and the (stationary) substrate is too small, it can be observed that the coating thickness in the area of the erosion pit is especially great, while it decreases greatly on either side of the erosion pit. As a result, such sputtering cathodes can be used only for coating substrates which have a much smaller diameter than the target.
Varian's "Con Mag" sputtering cathode, described above, springs from the attempt to improve the uniformity of the coating thickness by making not only the magnet systems but also the corresponding target parts completely separate electrically. This, however, requires the use of two current sources per sputtering cathode, and also electrical insulation of the target and cathode parts from one another.
A system of this kind is not suitable for sputtering targets of ferromagnetic materials.
The invention is therefore addressed to the problem of devising a sputtering cathode for both magnetic and nonmagnetic target materials, which will produce coatings of very uniform thickness and yet will be able to operate with only one source of electrical power.